Fix: Make CBillboardSceneNode bounding-box large enough to fit the billboard inside.

Fix: Make CBillboardSceneNode bounding-box large enough to fit the billboard inside.

It still won't work yet for scaled boundingboxes (or parents being scaled).
But at least it's now large enough for typical unscaled boundingboxes.
Before it was always too small - even for the simplest quadratic billboard case seen without rotation.
Now it's always a bit too large, but that's way less of a problem (collisions still work and culling simply happens a bit less often, but not too often which is way worse)

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@6431 dfc29bdd-3216-0410-991c-e03cc46cb475